This invention generally relates to III-V compound alloy semiconductor materials and devices and, more particularly, to such of those materials and devices as may be used for producing visible injection electroluminescence. This invention further relates to an improved method for use in producing compound alloy semiconductor materials and devices with a substantially reduced lattice constant mismatch.
At present the best known and most highly luminous near edge emission semiconductor materials and devices are made from the ternary GaAs.sub.1-x P.sub.x compound alloy system. Near edge emission is herein defined to include the emission resulting from radiative recombinations of electrons and holes across the energy band gap of the semiconductor material as well as radiative recombinations of electrons and holes originating and/or terminating on shallow defect states. The relative internal quantum efficiency .eta..sub.q, int of the GaAs.sub.1-x P.sub.x compound alloy system as normalized to .eta..sub.q, int of GaAs (hereinafter referred to as the normalized internal quantum efficiency), the photopic luminosity y.sub.10,.lambda. as normalized to its maximum value (hereinafter referred to as the normalized photopic luminosity), and their product .eta..sub..phi. (hereinafter referred to as the normalized luminous efficiency) are plotted in FIG. 1 as functions of composition, energy band gap, and wavelength of emitted light. Both .eta..sub.q, int and .eta..sub..phi. are plotted for three ratios (1, 10 and 100) of the electron recombination lifetime .tau..sup.X of excess carriers within the indirect conduction band minima to the electron recombination lifetime .tau..sup..GAMMA. of excess carriers within the direct conduction band minimum. This ratio is hereinafter referred to as the recombination lifetime ratio. It increases with the crystalline purity and perfection of the compound alloy system and conventionally ranges in value from 1 to 10, although values as high as 100 may be possible in the near future. See U.S. Pat. No. 3,398,310 issued to Larsen et al on Aug. 20, 1968.
As indicated in FIG. 1, the normalized internal quantum efficiency .eta..sub.q, int of the ternary GaAs.sub.1-x P.sub.x compound alloy system begins to decrease appreciably with increasing values of x at wavelengths ranging from about 7500 to 6600 A as the recombination lifetime ratio increases. These wavelengths are substantially longer than the wavelength of maximum photopic luminosity to the human eye (about 5500 A). Thus, it may be seen that for the recombination lifetime ratios 1, 10 and 100, normalized luminous efficiencies .eta..sub..phi. greater than 0.01 can only be obtained over a limited spectral range including wavelengths of about 6000 to 5700 A. Since the recombination lifetime ratio of the GaAs.sub.1-x P.sub.x compound alloy system currently ranges between 1 and 10, it may be seen that the highest normalized luminous efficiency .eta..sub..phi. presently is no greater than about 0.08 and is obtained from a direct composition (herein defined as a composition below the direct-to-indirect crossover point) at a wavelength of about 6500 A. As indicated for the recombination lifetime ratio 100, a normalized luminous efficiency .eta..sub..phi. as high as 0.2 may be obtained in the future from an indirect composition (herein defined as a composition above the direct-to-indirect crossover point) at a wavelength of about 6250 A. Moreover, a higher normalized luminous efficiency .eta..sub..phi. can currently be obtained at a wavelength as short as 6000 A from some indirect compositions having a recombination lifetime ratio greater than 1 (for example, 10) than can be obtained at a longer wavelength from any direct composition having a recombination lifetime ratio about an order of magnitude smaller (for example, 1). This high performance from indirect compositions of the GaAs.sub.1-x P.sub.x compound alloy system may be explained primarily on the basis of two factors, as set forth in greater detail in U.S. Pat. No. 3,398,310 mentioned above. First, the indirect compositions have greater energy band gaps than have the direct compositions of this compound alloy system. Secondly, the efficient, radiative direct recombination mechanism begins to successfully compete with the inefficient, nonradiative indirect recombination mechanism in indirect compositions near the direct-to-indirect crossover point (hereinafter referred to simply as the crossover point) as the crystalline purity and perfection of the system is improved enough to make the recombination lifetime ratio greater than unity. However, it is still presently estimated that even in the future electroluminescent semiconductor materials and devices fabricated from the GaAs.sub.1-x P.sub.x compound alloy system will only be capable of emitting red or orange light at a luminous efficiency no greater than 0.2 lumens per ampere.
Accordingly, the principal object of this invention is to provide improved near edge luminescing semiconductor materials and devices capable of emitting yellow and green light at a higher luminous efficiency, than can be obtained from the conventional ternary GaAs.sub.1-x P.sub.x compound alloy system.
Another more general object of this invention is to provide an improved semiconductor material that may be used for making transistors, Schottky-barrier diodes, optical modulators and detectors, injection electroluminescent light sources, photodetectors and other such devices.
Semiconductor materials and devices are commonly fabricated from the ternary GaAs.sub.1-x P.sub.x compound alloy system by epitaxially growing a layer of a selected composition near or at the crossover point of the system on a GaAs substrate. The lattice constant mismatch between GaAs and the crossover composition of the GaAs.sub.1-x P.sub.x compound alloy system is about 1.62 percent. This lattice constant mismatch in combination with the composition gradient produced during growth of the epitaxial layer of selected composition causes dislocations at the interface between the substrate and the epitaxial layer. For a lattice constant mismatch as large as 1.62 percent there is a strong probability that some of these dislocations will propagate into the epitaxial layer. Dislocations in the epitaxial layer give rise to recombination centers that compete with those of the desired near edge emission. They also cause p-n junctions subsequently diffused into the epitaxial layer to have a ragged diffusion front. These factors degrade the injection and luminous efficiency of the resultant semiconductor materials and devices.
Accordingly, still another object of this invention is to provide a method for epitaxially growing a semiconductor compound alloy on a substrate with a substantially reduced lattice constant mismatch between the substrate and the epitaxially grown compound alloy so as to minimize the dislocations produced at the interface between the substrate and the compound alloy.
These objects are accomplished according to the preferred embodiments of this invention by epitaxially growing a selected composition of the quaternary Ga.sub.y In.sub.1-y As.sub.1-x P.sub.x compound alloy system with values of x and y greater than 0.005 and less than 0.995 or a value of x equal to 1.0 and a value of y from 0.45 to 0.80 on a substrate having a lattice constant substantially equal to the lattice constant of the selected composition of the system. The selected composition of the system is grown by substantially following an isolattice constant contour of the system to the region of selected composition so that a layer of graded composition is formed upon the substrate and a uniform layer of the selected composition is formed upon the layer of graded composition. An injection electroluminescent diode having a high luminous efficiency may be fabricated from the resultant semiconductor material by diffusing a p-n junction into the uniform layer of selected composition and by forming electrical terminals in contact with the substrate and the uniform layer of selected composition on opposite sides of the p-n junction.